Sodium is a major positively charged electrolyte in the extracellular fluid. Its exchange with Potassium across cell membranes allows cells to maintain proper fluid balance; it also helps regulate blood pressure & acid-base balance (transmission of nerve signals, aids muscle contraction, & assists in the absorption of some nutrients). Sources: pickles, ham, chipped beef, tomato juice, soup & sauce, cream corn, potato chips, and saltine crackers.